912iS Shunt Regulator Experiment
There have been some issues with the 912iS stators, regulators and the wiring between, all to do with heat. It's something I have been interested in, so I did an experiment today and the results made me realize I made a misleading statement on this forum a while back. I could not find the old post, so I will make the correction here.
What I said was that the max wattage the shunt regulator can use (dissipate) is about 60 watts, or less than 15% of alternators rated load. This is true. But what I neglected to consider is the disproportionally high current (amps) associated with those watts. Amps is a measurement of electrical stress on components, the higher the voltage (pressure) is, the easier it is to push the same watts through a system. In other words, amps and volts are inversely proportional.
Today I put an inductive amp meter on one of the 3-phase leads that run from the stator to the regulator. I ran the engine at 3000 RPM and turned off all airframe loads. The amp meter indicated 21 amps! I then started adding loads to the airframe and the amps on the stator leads decreased progressively until everything was turned on. I have about 10 amps of avionics and lighting in the aircraft, and with it all on the meter on the stator lead was reading 13 amps.
I was initially surprised that there was 21 amps of shunting load with zero connected load downstream of the regulator. But then I realized this does reconcile with my understanding that 60 watts is the maximum the shunt regulator will ever need to dissipate. The reason is that those 60 watts are being delivered at a very low voltage during the shunting process (about 1.5 volts across the SCR's in the shunt).
Applying OHM's law for a 3-phase AC circuit:
21 amps x 1.5 volts x1.73 = 55.5 Watts.
Watts are power, so the engine is not working as hard to spin the alternator when no loads are connected, but the stress on the wiring (amps) due to the shunt regulator is higher. So, while 55 watts is only about 13% of the alternators rated power, the fact that it's imposing a high amperage on the wiring and stator is more significant when preservation of the components is the concern. Current (amps) create heat in the wiring, so I can see how a very lightly loaded airframe could contribute to heat damage to the regulator and those items upstream. I did not have a way to test this today, but I believe that with the alternator fully loaded at 420 watts (at 13.8 volts) the amps between the stator and regulator would be lower than with no load, about 18 vs 21. Also, it looks like with 10 amps (or more) of airframe load, the shunting load is not much of a factor.
